A nozzle structure attached to the lower end of a conventional plasma torch, as shown in FIG. 2, comprises a nozzle 10, an annular insulation member 30, an annular swirler 40, and an electrode 50. The nozzle 10 has an upper cylindrical portion, a frustoconical portion, a lower cylindrical portion and a projecting conical lower end portion with an end face, forming an interior chamber. The end face of the nozzle 10 has a hole therein in communication with the interior chamber of the nozzle 10 for confining a plasma jet. The swirler 40 is positioned within the interior chamber of the nozzle 10, resting on the shoulder between the upper cylindrical portion and the frustoconical portion of the nozzle 10. The electrode 50 is positioned by the swirler 40 and the insulation cylinder 30 to be coaxial with the nozzle 10. The swirler 40 jets working gas in the form of a swirl steam or an axial stream into an annular gap between the electrode 50 and the nozzle 10. A cap 20 has an upper cylindrical portion and a lower conical portion, with the lower conical portion having a hole centrally formed therein. The cap 20 is positioned about and coaxially with the nozzle 10 with the sides of the hole in the lower end of the cap 20 being in close contact with the conical surface of the lower conical portion of the nozzle 10, so that the outer surface of the nozzle 10 is encompassed by the cap 20, excepting only the lower end face of the nozzle 10. A space defined between the outer periphery of intermediate portions of the nozzle 10 and the inner periphery of the cap 20 serves as a passageway 70 for nozzle cooling water. The cylindrical portion of the cap 20 is secured to the cylindrical portion of the torch body 80 by suitable means.
The cap 20 serves the function of securing to the torch body 80 the nozzle 10 and the components fitted therein, as well as ensuring electrical conduction to the nozzle 10. In such a plasma torch in which the nozzle 10 is directly cooled by water, the cap 20 also serves the function of sealing the cooling water passageway 70 so as to prevent the cooling water flowing around the outer periphery of the nozzle 10 from leaking out of the torch.
In order to achieve these functions effectively, the cap 20 is made to have metal-to-metal contact with the nozzle 10 in the vicinity of the lower (front) end of the plasma torch, and the diameter of the cylindrical portion of the cap 20 and the apex angle .theta..sub.2 of the conical portion of the cap 20 are both made to be large. The large diameter of the cylindrical portion of the cap 20 and the large apex angle of the conical portion of the cap 20 readily accommodate the various components positioned in the vicinity of the front end of the plasma torch, e.g., the insulation cylinder 30 fitted on the outer periphery of the electrode 50, the swirler 40 attached to the lower end of the insulation cylinder 30, the nozzle 10 in which these components are fitted, and the like.
Such a plasma cutter can be used in two-dimensional cutting of a workpiece such as a flat plate, and can also be used in three dimensional cutting, e.g., for forming components of automobiles or the like by installing the plasma cutter onto a robot. However, the conventional plasma torch having a large diameter cap and a large apex angle .theta..sub.2 at the lower end of the torch has incurred the following problems.
(1) If the height of the plasma torch (the distance between the lower surface of the nozzle 10 and the cutting surface on the workpiece) is set to a predetermined desired value, the sides of the lower end of the torch can sometimes make contact with the workpiece when the cutting is being conducted in a trough-like recess in the workpiece. Accordingly, it is not always safe for the height of the torch to be maintained at the predetermined desired value. That is, the accessibility of the plasma torch is inferior due to the size and shape of the lower end of the nozzle structure. Where the cutting has to be made with a torch height greater than the predetermined desired height, the cutting quality at that location can be extremely poor, and in some cases no cutting can be made.
(2) During a robotic teaching work wherein cutting loci are being stored in memory in a robot, a marking line made on the workpiece to be cut can be hidden by the torch so that the teaching work becomes difficult.
(3) Since the portion of the cap 20 which contacts the nozzle 10 is in close proximity to the lower end of the nozzle 10, molten metal which splatters during a cutting operation can contact the cap 20 and adhere thereto. Thus, an abnormal electric discharge or a double arc point is likely to occur. Accordingly, damage to the cap 20 can occur, thereby causing leakage of cooling water through the metal-to-metal seal between the nozzle 10 and the cap 20.